Process for the production of a hard metal of increased toughness



United States Patent PROCESS FOR THE PRODUCTION OF A HARD METAL 0F INCREASED TOUGHNESS Victor Berg, Kapfenberg, Austria No Drawing. Appiication December 21, 1951, Serial No. 262,883

Claims priority, application Austria December 27, 1950 2 Claims. (Ci. 148--13) Hard metals used for machining are required to possess a high toughness in addition to a high hardness mostly exceeding 1500 Vickers DPH units. Several methods have already been proposed to enhance the toughness of hard metals. Such are methods of alloying, e. g. the partial substitution of titanium carbide by the carbides of niobium and tantalum in hard metals composed of tungsten carbide and titanium carbide or an increase in the amount of auxiliary metal added, which, however, may be used to a certain limit only without decreasing the hardness to a disadvantageous extent. Another method of increasing the toughness comprises the use of two or more grain sizes of hard component instead of a uniformly fine grain, by these means increasing the thickness of the films of auxiliary metal between the individual grains of the hard component. This method has, however, equally limitations as an excessive increase of these films leads to a decrease of the hardness even without increasing the amount of auxiliary metal present. Thus extensive investigations in each individual case are required to establish the method of production and the composition of the hard metal best suited.

The present invention proposes an additional method of increasing the toughness of hard metals. Whereas it was hitherto thought impossible to influence the properties of hard metals by heat treatment, a close study of the phenomena occurring during cooling from the sintering temperature revealed that the toughness of the auxiliary metal phase was by no means unchangeable. This, however, is of course of extreme importance for the toughness of the hard metal. In the auxiliary metal phase hard components as e. g. the carbides of tungsten, titaniurn etc. are dissolved during sintering. These may be caused to precipitate by a suitable heat treatment. Under conditions of cooling from sintering temperature normally prevailing this precipitation is not complete and considerable amounts of hard components such as carbides remain dissolved in the binder phase and increase the hardness with simultaneously decreasing its toughness.

The heat treatment to follow the sintering process according to the present invention intended to precipitate the dissolved hard components out of the binder phase consists in annealing at temperatures between 600 and "ice i000 C., preferably between 800 and 900 C. for 2 to 10'hours.

In the case of cobalt being used as an auxiliary metal and tungsten carbide for a hard component the hardness of the binder phase normally attains approximately RC units whereas the hardness has decreased to 38 RC after annealing for 2 hours at a temperature of 900 C. The progress of heat treatment can be determined by measuring the amount of alteration of the specific magnetisation of the cobalt, the specific magnetisation amounting to approximately in the as sintered and without heat treatment state and exceeding cgs. units after the heat treatment according to the present invention.

The heat treatment of the present invention can be favourably applied to hard metals having an increased toughness obtained by grading the grain size of the hard component in a known Way. It should, however, be remembered that a binder phase of decreased hardness and increased toughness will be obtained, consequently it will mostly be suitable to consider this fact prior to selecting the grain size.

According to the present invention the grain size of the hard component should preferably consist of a fine portion of a grain size of 0.5 to 2 and a coarse portion of 2 to 5 the respective amounts being selected in the ratio of 1:3 to 3:1. 7

On using the principles embodied in the present invention hard metals are obtained with a considerably increased resistance to shock.

. I claim:

1. The process of toughening the binding matrix of a hard metal carbide selected from the group consisting'of titanium, columbium, tantalum and tungsten, sintered in a matrix of cobalt metal as a mixture of fine particles having an average grain size of 0.5 to 2 microns and coarse particles having an average particle size of 2 to 5 microns, the ratio of fine to coarse particles being in the range of 1:3 to 3: 1, comprising heating the sintered product at a temperature in the range of 800 to 900 C. for a period of two to ten hours to allow dissolved hard carbide to precipitate.

2. The process of toughening the binding matrix of a hard metal carbide selected from the group consisting of titanium, columbium, tantalum and tungsten, sintered in a matrix of cobalt metal as a mixture of fine particles less than about 5 microns in size, comprising heating the sintered product at a temperature in the range of 800 to 900 C. for a period of tWo to ten hours to allow dissolved hard carbide to precipitate.

References Cited in the file of this patent UNITED STATES PATENTS 2,066,515 Bishop Jan. 5, 1937 2,285,909 Dawihl June 9, 1942 2,313,070 Henzel Mar. 9, 1943 2,349,052 Ollier May 16, 1944 2,486,576 Savage Nov. 1, 1949 

1. THE PROCESS OF TOUGHENING THE BINDING MATRIX OF A HARD METAL CARBIDE SELECTED FROM THE GROUP CONSISTING OF TITANIUM, COLUMBIUM, TANTALUM AND TUNGSTEN, SINTERED IN A MATRIX OF COBALT METAL AS A MIXTURE OF FINE PARTICLES HAVING AN AVERAGE GRAIN SIZE OF 0.5 TO 2 MICRONS AND COARSE PARTICLES HAVING AN AVERAGE PARTICLE SIZE OF 2 T 5 MICRONS, THE RATIO OF FINE TO COARSE PARTICLES BEING IN THE RANGE OF 1:3 TO 3:1, COMPRISING HEATING THE SINTERED PRODUCT AT A TEMPERATURE IN THE RANGE OF 800 TO 900*C. FOR A PERIOD OF TWO TO TEN HOURS TO ALLOW DISSOLVED HARD CARBIDE TO PRECIPITATE. 